The present invention relates to integrated circuit devices and their fabrication.
High performance circuits, especially those used for radio frequency chips, favor the use of heterojunction bipolar transistors (HBTs) to provide high maximum oscillation frequency fMAX and high transit frequency fT, also referred to as “cutoff frequency”. HBTs have a structure that includes a junction formed by juxtaposing two dissimilar semiconductors. For example, an HBT may have a base layer including a semiconductor alloy material such as silicon germanium (SiGe), having substantial germanium content and profile, juxtaposed to a collector region of silicon or an emitter layer of polysilicon.
An advantage of an HBT is that a heterojunction can be designed to have a large current gain. Increased current gain permits the resistance of the base to be decreased by allowing a higher dopant concentration to be provided in the base of the transistor. To increase the performance of an HBT, it is desirable to increase both the transit frequency fT and the maximum oscillation frequency fMAX. FMAX is a function of fT, parasitic resistances and parasitic capacitances (both collectively referred to herein as “parasitics”) between elements of the transistor according to the formulafMAX=(fT/8π*Ccb*Rb)1/2.
The parasitics of the HBT include the following parasitic capacitances and resistances, as listed in Table 1:
TABLE 1Ccb collector-base capacitanceCeb emitter-base capacitanceRc collector resistanceRe emitter resistanceRb base resistance
The most significant parasitics are the collector-base capacitance Ccb and the base resistance Rb, because they provide an electrical feedback path between the output and input of the transistor, reducing power gain and thus reducing gain-dependent figures of merit including fMAX. Their values are typically larger than the other parasitics, making their effects on fT and fMAX more pronounced. Thus, it is desirable to provide an HBT structure and method by which Ccb and Rb are significantly reduced.
An example of a state of the art heterojunction bipolar transistor (HBT) structure containing parasitics is illustrated in FIG. 1. As depicted in the cross-sectional view therein, an ideal or “intrinsic” device consists of a one-dimensional slice downward through the centerline 2 of the HBT, through emitter 4, intrinsic base layer 3, and collector 6. The emitter 4 is generally heavily doped with a particular dopant type, (e.g. n-type), and generally consists essentially of polycrystalline silicon (hereinafter, “polysilicon”). The intrinsic base 3 is predominantly doped with the opposite type dopant (e.g. p-type), and less heavily than the emitter 4. The collector 6 is doped predominantly with the same dopant (e.g. n-type) as the emitter 4, but even less heavily than the intrinsic base 3. Region 5 represents the depletion region disposed between the intrinsic base 3 and the collector 6, due to the p-n junction between the base and collector, which have different predominant dopant types. Region 7 represents the depletion region disposed between the intrinsic base 3 and the emitter 4, due to the p-n junction between the base and emitter, which have different predominant dopant types. Often, the intrinsic base 3 is formed of silicon germanium (SiGe), which is epitaxially grown on the surface of the underlying collector 6.
The ideal structure itself contains two capacitances that impact performance. Intrinsic emitter-base capacitance CBE,I arises at the junction 7 between the emitter 4 and the base 3. In addition, there is an intrinsic collector base capacitance CCB, I at the junction 5 between the collector region and the base. The values of these capacitances are related to the areas of the respective junctions, as well as to the quantities of dopant on either side of the respective junctions. Although these capacitances impact the power gain of the transistor, they are an inextricable part of the ideal transistor structure and thus cannot be fully eliminated.
Unfortunately, a one-dimensional transistor, which is free of all material beyond the intrinsic device, cannot be realized in a practical process. A typical transistor contains additional parasitics stemming from interaction between the intrinsic device and other material structures in which the intrinsic device is embedded. Such material structures help provide electrical access to and heat transfer away from the intrinsic device. One such parasitic having a key impact upon power gain is the extrinsic collector base capacitance CCB, E. As shown in FIG. 1, CCX and CRX are components of the extrinsic collector base capacitance CCB, E. The first component capacitance CCX results from interaction between the extrinsic base of the device and the collector pedestal. The second component capacitance CRX results from interaction between the extrinsic base of the device and the bulk substrate portion of the collector, between the edge of a shallow trench isolation (STI) 9 and the collector pedestal 6. An additional component capacitance CPB is the capacitance of the extrinsic base and substrate where separated by the STI. Ideally, the fabrication process of an HBT results in an STI having a thickness which is sufficient to avoid substantial CPB. In such case, the parasitic capacitances CCB, I, CCX and CRX contribute more significantly to the overall collector base capacitance Ccb than CPB.
As illustrated in FIG. 2, the extrinsic base resistance Rb is a second important parasitic. Rb represents the series resistance between the external base contact and the intrinsic base film. The components of the base resistance Rb include: Rint, which is a function of the size of the emitter and the intrinsic base profile. Another component, Rsp+link, is a function of the width of the spacer separating the raised extrinsic base layer from the emitter, and is also a function of the interface quality of the link between the intrinsic base and the raised extrinsic base. Another component is Rpoly, which is function of the thickness, doping and alignment of the edge of the silicide 11 (when present) to the polysilicon layer 8 of the raised extrinsic base. Rsilicide, is a component which is a function of the dimension of the polysilicon over which the silicide 11 is disposed. The parasitic resistances Rpoly and Rsilicide contribute significantly to overall base resistance Rb.
Typically, moving the extrinsic base elements closer to the intrinsic device reduces Rb. However, such an approach tends to increase the extrinsic collector base capacitance CCB, E, creating a fundamental tradeoff between the two parasitics and making it hard to improve overall power gain. Narrowing the collector pedestal itself can also reduce CCB, E. Such a reduction is difficult to achieve, however, since the pedestal is typically formed by implantation of dopants, which tend to scatter laterally during implantation and to diffuse laterally during the typical heating that a transistor experiences during fabrication. Narrowing the collector pedestal also increases the collector resistance (RC) of the collector pedestal, impacting high frequency performance. Thus, it is desirable to avoid narrowing the collector pedestal.
A structure and method of confining the lateral dimension of the collector pedestal near the point of interaction with the extrinsic base, while maintaining low RC and preserving tolerance against process thermal cycle would be of major advantage in improving the high-frequency gain of a bipolar transistor.
Therefore, it would be desirable to provide a structure and method of fabricating a bipolar transistor having reduced extrinsic collector base capacitance CCB,E without significantly impacting the extrinsic emitter base resistance Rb or the collector resistance RC, so as to achieve superior high-frequency power gain.
Commonly assigned, co-pending U.S. patent application Ser. No. 10/249,299 describes an HBT having reduced collector-base capacitance and resistance, by vertically interposing first and second shallow trench isolation (STI) structures between the collector, which underlies the STI, and the raised extrinsic base which overlies the STI.
It would further be desirable to increase the transit frequency fT and maximum oscillation frequency fMAX through change in one or more of the vertical profiles of the collector, base, emitter and/or the junctions between them.